Apparatus and method for measuring work



June 12, 1945. ELLIQTT APPARATUS AND METHOD FOR MEASURING WORK Filed NOV. 1'7, 1942 2 Sheets-Sheet l 5 2 2 f R om 0 0 H M U T O 0 0 M 5 A R R m m V/ICUVM 7' USE VOL 7' ME 7' ER gwuwvbo b MYRON A. ELLIOTT 7'0 OSC/ZZHTOR/O June 12, 1945. M. A. ELLIOTT 2,377,869

APPARATUS AND METHOD FOR MEASURING WORK Filed Nov. 1'7, 1942 2 Sheets-Sheet 2 MYRON A. ELLIOTT Patented, June 12, 1945 srrsas'ros sup METHOD roa amssonmo woax Myron A. Elliott, Washington, D. 0.

Application November 17, 1942, Serial No. 465,938

9 Claims. (Cl. 73-98) (Granted under the m of March a, 1883. as amended April so, 192:; m o. o. m)

This invention relates to apparatus for measuring a variable force over a period of time, and it is more particularly directed to an apparatus and method for measuring this force over a distance through which it acts. and applications of the method to the testing of materials.

In the testing of many materials it is customary to apply a force to the material until failure occurs or the material yields to some degree. In such cases it is often diflicult, if not impossible, to apply a steady force or a uniformly increasing one, or to control the rate of yield due to irregularities in the specimen. Where the force varies greatly or where accurate measurements are desired the usual balance is not a satisfactory indicator of the force because, first, it will not give an average reading or a summation of readings over a period of time and, second, the movement of the balance itself is suilicient to vary the rate yield to such an extent that the readings are not accurate or reproducible.

This invention provides an apparatus for measuring such forces with a high degree of accuracy, and since it does not appreciably aifect the rate of yield of the specimen under test it is possible to control the applied force or rate of yield within narrow limits. This apparatus measures the force directly, or as it is applied over a predetermined period of time, either as an average value or as an integral, depending on whether the peri- 0d of time during which the force'is applied is taken as unity or as a sum of smaller units. A modification of the invention includes means for controlling the rate of yield of the specimenso that, in this case, the apparatus measures either the average force applied over a given distance or .the integral of the force taken over the same distance, which is the measure of the work done on the specimen. The invention also includes a specially designed multiple condenser for use in the measuring apparatus. and a method for determining propertie of a material by means of this apparatus. In particular, the method includes the testing of the deep drying properties of insulating varnishes.

The invention involves causing the force applied to the specimen to produce an electric signal as a function thereof, the signal so produced be- Fig. 2 is a diagram of a variation of the capacitance bridge of Fig. l, utilizing a two-section force measuring condenser;

Fig. 3 is a diagram of another variation of the capacitance bridge of Fig. 1, designed to use a four-section force measuring condenser;

Fig. 4 is a diagram of a four-section force measuring condenser to be used in the circuit of Fig. 3;

Fig. 5 shows a modification of the four-section condenser of Fig. 4; v

Fig. 6 shows a variation in the structure of the force measuring condenser shown in Fig. 4; and

Fig. 7 shows another variation in the structure of the force measuring condenser shown in Fig. 4.

Referring to Fig, 1, an oscillator in generates an alternating current signal which i applied to a capacitance measuring bridge il in conventional manner. The frequency of the signal'is not important, although a frequency of about 1000 cycles is usually employed for bridges of'this type. One side of the "unknown" leg of the bridge II is connected to a fixed metal bar it. The other side of the unknown" leg is connected to a metal bar It which is rigidly supported at one end by an insulating block it and positioned opposite the bar ii. In this way the bars I! and II form a variable condenser C1, the capacitance of which is measured by the bridge Ii. A small padding condenser II is connected across the bars I! and II to aid in balancing thebridge as described further on. The bar I2 carries avernier adjustment device consisting of a smaller metal bar It connected by means of adjustable screws II. By varying the spacing between the bar l3 and bar II the capacitance measured by bridge I I may be adjusted.

A sample II to be stretched, torn, or otherwise subjected to failure is fastened between ing measured by indicating means (or it may be clamps II and 20 which may be of any suitable design. The clamp is is suspended from the bar II, and a rope or belt 2|, fastened to the clamp.

of the bar it is proportional to the tension to which the sample I I is subjected, as the tension is not allowed to deflect the bar I. past its elastic limit.

In order to avoid excessive play in testing the sample ll, the bar II is preferably of sufficient rigidity that its deflection for the maximum tension to be measured is not greater than about $4 of the free lenEth of the bar ll, although this is only an approximate value.

When the sample ll is in place and ready for testing, the bridge ii is balanced, using the padding condenser ii if necessary to bring the voltage appearing across terminals lid and lib to zero. When the motor 23 is started the bar I! deflects away from the bar l2 according to the resistance to separation of the sample l4, thus unbalancing the bridge ii and causing a voltage to appear across the terminals I la and l lb. This voltage is approximately proportional to the deflection of the bar I! within the limits employed.

The voltage across the terminals Ila and lib is supplied to the input of a linear amplifier 24, the output of which is rectified in the rectifier 25, and filtered by a condenser 26. A load resistance 21 is connected across the output of the rectifier 25 so that variations in the indicating circuit load will not disturb the operation of the amplifier 24. A direct current voltmeter 2| is also connected across the rectifier output to aid in adjusting the gain of the amplifier 24, but it is not essential. From the load resistance 21 the rectifier output voltage is connected across a resistance 2! in series with a condenser ill, through the contacts of a relay ll. The relay II is energized by any convenient electrical source in series with a time delay witch 32. A voltmeter I3 is connected across the condenser 30 to measure the total charge thereof. Since appreciable current cannot be drawn from the condenser it without greatly reducing the charge, the voltmeter 38 should be of the vacuum tube type, or it may be a heavily damped galvanometer of the type known as a coulomb meter or ballistic galvanometer. If desired, the output of the bridge can also be measured directly without rectification by the use of a watt-hour type of meter.

The resistance 20 and the condenser 40 constitute the measuring part of the apparatus and their values are selected so that the condenser 30 can be charged to only a small fraction of the output voltage appearing across the load resistance 21 under maximum deflection of the bar I! during the time interval over which the tension on the sample II is to be measured. The particular values of the resistance 2! and the condenser will depend on a number of obvious factors, but for a maximum rectifier output of about 30 volts it has been found that a resistance of 15 megohms for the resistance 20 and a capacity of 50 microfarads for the condenser 14 permits the condenser II to accumulate a charge of about one volt in about 25 seconds. The rate at which the condenser ill charges is proportional to the voltage drop across the resistance 29, which, in turn, equals the voltage developed across the load resistance 21 minus the charge on the condenser 34. Therefore, a total maximum charge of about one volt on the condenser II is small enough to be neglected, and it may be said that the charge on the condenser SI is proportional to the product of the rectifier output voltage appearing across the load resistance 21 and the charging time. Thus if the apparatus is calibrated with the time interval at which the delay switch 32 operates, as unity, the apparatus measures the average value of force applied to the specimen. However, the apparatus may be calibrated for any time interval shorter than the period of the delay switch 32, and the resulting charge on the condenser 34 will be a summation of the force applied over the period of the delay switch 82, or a measure of the work done on the sample if the rate of yield is constant. By making the calibration time interval small the charge on the condenser 84 becomes the integral of the force applied. taken over the given period of time, or through a given yield distance of the specimen.

It is obviously not necessary that the amplification of the amplifier 24, or the change in capacitance of the condenser formed by the bars I! and It with applied force, be linear, because the apparatus is most easily calibrated by suspending a known weight from the bar It and observing the charge accumulated by the condenser in a given period of time, and for a given setting of the amplifier gain.

Referring to the Figs. 1 and 2, a variation 6: the capacitance bridge 11 of Fig. i is shown which develops a greater signal voltage for the same deflection of the bar ll. Instead of employing only one variable condenser, the bridge 4| has two condensers one in each of two adjacent legs, the effective positions of which are indicated by dotted lines and condensers 42 and 41. The condensers 42 and 48 are formed by a two-section variable-condenser C2, which is designed tovary the capacitances of the condensers 4! and 42 in opposite directions as a bar 44 is deflected. That is, the condenser Ca comprises two rigid metal bars or plates 44 and, 4 between which is the flexible bar 44 separated by insulators 41. As the bar 44 is deflected toward the bar 44 it moves away from the bar 48, thus increasing the capacitance of the condenser 42 and decreasing that of the condenser 43. Thus the bridge U is moved further out of balance than it would be if the condenser 48 remained fixed, and a proportionately larger signal is transferred to the amplifier 24. Where exact balancing of the bridge 4| is necessary, padding condensers may be placed in the circuit where the condensers 42 and 48 are shown.

Referring to Figs. 1 and 3, another variation of the capacitance bridge ll of Fig. l is shown which develops the maximum signal voltage across the input of the amplifier 24, and is consequently more sensitive than either of the bridges ii and 4| already described. In Fig. 3 the bridge ll contains four variable condensers, one in each leg thereof, the effective positions of which are shown by the dotted lines and condensers I2, 43, I4, and II. Where accurate balancing of the bridge is desired. two padding condensers may be inserted in the circuit where the condensers I2 and II, or 44 and II, are shown Only two padding condensers are necessary, although they could be placed in all four legs. In addition it is sometimes necessary to balance out a resistive component, and this may most, easily be done by inserting two variable resistances l4 and 61 shown in dotted lines. The four condensers l2, l8 l4, and II are formed by a four-section variable condenser, two variations of which are shown as condensers C: and C4 of Figs. 4 and 5 respectively. The roper connections between the bridge II and the condensers Ca and C4 are made by connecting the lines a, b, c.

. elements Cl and Ii.

and d or m. 3 with the correspondingly lettered lines Figs. 4, 5, 8, or '7.

Referring to Fig. 4, two flexible elements it and II are spaced from three rigid elements ll, 03, and N by insulators II at one end. At the other end the elements I and Ii are held in equal spacing by an insulating clamp It so that as the element ti is deflected, the element 60 is equally deflected in the same direction. By connecting the elements 6! and 04 together a four section condenser C: is formed, alternate sections of which vary in capacitance in the same direction and adjacent sections of which vary in capacitance in opposite directions, on deflection of the Therefore, by connecting the condenser C: in the bridge circuit of Fig. 3 as shown, a capacitance bridge is formed which is thrown the farthest out of balance (and hence gives the greatest signal to the amplifler 24) for any given deflection oi the elements II and II caused by the force being measured.

Referring to Fig. 5, a modification of the toursection condenser C: of Fig. 4 is shown as a condenser Ce. In this modiflcation two flexible ele ments II and Ii are placed opposite each other, each between rigid elements I2, I3 and 14, II respectively. The respective elements are supported and separated irom one another by insulators It. The flexible members II and Ii are deflected in the same direction simultaneously by means of an insulating clamp 11,,when the force to be measured'is appliedth'ereto. With the elements I! and II, and I8 and I4, respectively, connected together, as is shown in Pig. 5, a toursection condenser is formed, opposite sections of which vary in capacitance in the same direction and adjacent sections vary in capacitance in opposite directions as a force is applied to the clam II and the elements II and H are deflected. The condenser C4 is connected to the bridge ll of Fig. 8 in the same manner as described with respect to the condenser C: or Fig. 4.

In Fig. 6 there is shown a variation in construction oi! the four-section condenser C: of Fig. 4. Where it is undesirable to construct the flexible elements or the condenser of suflicient stiffness to keep from excessive bending when the force is applied thereto, it is convenient to have some other elastic device take up the force and merely have the condenser capacitance vary in accordance with the strain of the elastic device as it the condenser elements were absorbing the force themselves. This is easily accomplished in a number of ways. As shown in Fig. 6, stationary elements II, It, and N are supported by insulators N in the same way as the elements '2, ll, and N are supported by the insulators it in Fig. 4. However, movable elements It and II, which correspond to the flexible elements It and II of Fig. 4 are separated by an insulator It and supp rted from a flxed member II by a relatively heavy spring ll, or other suitable resilient device. When a force is applied to the members II and Ii they will move toward and away from the respective stationary members in the manner described with respect to Fig. 4, thus accomplishing the same result.

In Fig. 7 there is shown another variation in construction of the tour section condenser Ci of Pig. 4. In this variation the Spring element, or elastic member, which absorbs the force applied to the condenser comprises a pair oi flexible leaves II which are fastened to a rigid element l1 and to an insulating support I. As is obvious (mm the drawings, theremaining part of the concausing the application of the force to the elements I! and ii or ll and .II to be reversed. Similarly the condensers of Figs. 1. 2, and 5 may be constructed with separateelastic means for taking up the applied force, thus eliminating the need for flexible elements in the condenser. Also,

7 by this construction, the condenser may be made of extremely light materials.

Many of the various uses or the apparatus described and method of measuring forces therewith will be apparent. However, the apparatus was particularly developed in connection with a method for measuring the deep-dryin properties of insulating varnishes. This method comprised coating two flexible sheets, such as flannel or other tightly woven cloth, with the varnish to be tested, placing them together under predetermined-pressure (usually 10 lbs. per square inch) between steel slabs with copper, foil placed between the cloth and the steel, and baking the varnish under this pressure under controlled conditions of time and temperature. After baking. the laminated cloth was removed from between the slabs and the copper foil peeled off. The two pieces of cloth were then slightly separated and inserted between the clamps II and I. of Fig. 1 and slowly drawn apart at a predetermined speed. The average force required to separate the pieces of cloth, which was measured in the mannerdescribed with respect to Fig. 1, was a measure of thedegree of hardening or "deep-drying" of the varnish away from exposed edges of the sample.

Many other variations in the apparatus described and methods in which it may be employed willbeobvious tothoseskilledintheart,andthe invention should not be limited other than as deflned by the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment or any royalties thereon or therefor.

I claim:

1. Apparatus for measuring a force which comprises, a capacitance bridge including at least one variable condenser, means for unbalancing said bridge by varying the capacitance of said variable condenser in accordance with said force, an alternating current signal source connected to said bridge, converting means connected across said bridge so as to convert the signal produced by unbalance or said bridge into a direct current signal, a resistance and a second condenser connected in series to said converting means to receive said direct current signal, means for controlling the time duration oi. said signal, and means connected across said second condenser to measure'the accumulated charge thereon.

2. Apparatus for measuring a force which comprises, a capacitance bridge including a variable condenser in each arm thereof, means for unbalancing said bridge by v ryi g the capacitance oithecondensersinoppositearmsthereotintbe same direction and varying the capacitance of the condensers in adjacent arms in opposite directions in accordance with said force, an alternating current signal source connected to said bridge, converting means connected across said bridge so as to convert the signal produced by unbalance of said bridge into a direct current signal, a resistance and a second condenser connected in series to said converting means to receive said direct current signal, means for controlling the time duration of said signal, and means connected across said second condenser to measure the accumulated charge thereon.

3. Apparatus for measuring a force which comprises, a balanced capacitance bridge including at least one variable condenser, said condenser com prising a plurality of conducting surfaces positinned opposite one another and including at least one stationary member and one movable member, means for applying a force to said movable member to cause it to move toward or away from said rigid member thereby to change the capacitance of said condenser and unbalance said bridge in accordance with said force, an alternating current signal source connected to said bridge, an amplifying channel connected across sair bridge and a rectifier connected to the output of said amplifying channel to produce an amplified direct current signal according to the unbalance of said bridge, a resistance, a second condenser and a time switch connected in series to said rectifier to receive said amplified direct current signal for a predetermined period of time, and means connected across said second condenser to measure the accumulated charge thereon.

4. Apparatus for measuring a force which comprises, a balanced capacitance bridge including four variable condensers one in each arm thereof said variable condensers comprising a plurality of conducting surfaces including at least three stationary members and two movable members positioned between said stationary members; means for applying a force to said movable members to cause them to move toward at least one stationary member thereby to unbalance said bridge in accordance with said force, an alternatin current signal source connected to said bridge, an amplifying channel connected ross said bridge and a rectifier connected to the output of said amplifying channel to produce an amplified direct current signal according to the unbalance of said bridge, a resistances. second condenser and a time switch connected in series to said rectifier to receive said amplified direct current signal for a predetermined period of time, and means connected across said second condenser to measure the accumulated charge thereon.

5. Apparatus for measuring a variable force over a given distance through which it acts which comprises, a balanced capacitance bridge including at least one variable condenser, said condenser comprising a plurality of conducting surfaces positioned opposite one anothe and including at least one rigid member and one flexible member, tension means for applying said force to said flexible member to cause said flexible member to bend toward or away from said stationary member thereby to change the capacitance of said condenser and unbalance said bridge; said tension means comprising a first clamp connected to said flexible member and adapted to grip one end of a specimen to be tested, a second clamp adapted to grip the opposite end of said specimen, and power means cooperating with said second clamp and adapted to draw said second clamp away from said first clamp at a predetermined, substantially uniform rate; an alternating current signal source connected to said bridge, an amplifying channel connected across said bridge and a rectifier connected to the output of said amplifying channel to produce an amplifled direct current signal according to the unbalance of said bridge, a resistance, a second condenser and a time switch connected in series to said rectifier to receive said amplifled direct current signal for a predetermined period of time, and means connected across said second condenser to measure the accumulated charge thereon 6. Apparatus as claimed in claim 5 whereinaaid balanced bridge contains a variable condenser in each arm thereof; said variable condensers comprising a plurality of conducting surfaces including at least three stationary members and two flexible members positioned between said stationary members, and said tension means is adapted to bend said flexible members toward or away from said stationary members.

7. A multiple variable condenser adapted for measuring forces applied thereto when connected in a capacitance bridge which comprises, four capacitance sections, said four sections comprising three stationary conducting surfaces and two flexible conducting surfaces positioned between said stationary surfaces, the outside stationary members being connected together to form said capacitance bridge so that, on deflection of said flexible surfaces in the same direction alternate sections of said bridge change capacitance in the same direction and adjacent sections change capacitance in opposite directions.

8. A multiple variable condenser adapted for measuring forces applied thereto when connected in a capacitance bridge which comprises four capacltance sections arranged in two groups of two each. each group comprising two stationary conducting surfaces and one flexible conducting surface therebetween, the corresponding surfaces of said sections in each group lying substantially in the same plane, said stationary surfaces of each group connected to the stationary surfaces of the other group lying on the opposite side of said flexible surfaces to form said capacitance bridge so that, on deflection of said flexible surfaces in the same direction alternate sections in said bridge change capacitance in the same direction and adjacent sections change capacitance in opposite directionsv 9. Method of measuring a force over a given period of time which comprises causing said force to unbalance a capacitance bridge by applying said force to a movable surface of a condenser in said bridge, amplifying the signal caused by said unbalance, converting said amplified signal to a direct current signal, charging a flxed condenser through a resistance by means of said direct current signal for a predetermined period of time and measuring the accumulated charge on said fixed-condenser.

MYRON A. ELLIOTT. 

